2626
Ind. Eng. Chem. Res. 2006, 45, 2626-2633
Diethylenetriamine[propyl(silyl)]-Functionalized (DT) Mesoporous Silicas as CO2 Adsorbents Gregory P. Knowles, Seamus W. Delaney, and Alan L. Chaffee* School of Chemistry, Monash UniVersity, Victoria 3800, Australia
Mesoporous silica substrates were functionalized with N-[3-(trimethoxysilyl)propyl]diethylenetriamine to form diethylenetriamine[propyl(silyl)]- (DT-) functionalized hybrid products suitable for CO2 adsorption. The materials prepared were characterized by N2 adsorption/desorption at 77 K, C and N elemental analysis, helium pycnometry, X-ray diffraction (XRD), CO2 adsorption, and thermal decomposition and were compared to analogous aminopropylsilyl- (AP-) and ethylenediamine[propyl(silyl)]- (ED-) functionalized materials. The extent of surface functionalization varied with substrate morphology. CO2 adsorption capacities and heats of adsorption were determined via combined thermogravimetric analysis and differential thermal analysis (TGA/ DTA). Functionalization of the substrates was found to enhance their CO2 adsorption capacities at 20 °C under anhydrous conditions. Higher temperature led to reduced adsorption capacities but higher heats of adsorption (Hads) of CO2, thought to be due to the reduced role of weak physisorption sites. When CO2 was supplied in a moist gas stream, the adsorption capacity was reduced, but the value of Hads(CO2) was essentially unchanged. The thermal stabilities of one substrate and its AP-, ED-, and DT-functionalized products both in N2 and in mildly oxygenated N2 were also characterized by combined TGA/DTA. These materials were found to be stable up to 170 °C in both atmospheres and, furthermore, had no particular affinity for either N2 or O2 over this temperature range. Oxidative decomposition data from TGA at higher temperature were found useful for estimating the N content of these materials. Introduction There is widespread interest in the reduction of greenhouse gas emissions from fossil fuel combustion, particularly from significant point sources such as coal-fired power stations, because of the potential negative impact global climate change might have on humanity. The development of high-capacity CO2-selective adsorbents that could be employed in vacuum swing adsorption (VSA) processes to capture and then sequester the CO2 product is being considered as one way to achieve this goal. Previously, we considered aminopropylsilyl- (AP-) and ethylenediamine[propyl(silyl)]- (ED-) functionalized hexagonal mesoporous silicas (HMSs) as adsorbents for the selective capture and separation of CO2 from flue gas streams1 (illustrated in Figure 1). The presence of the surface-bound amine groups is thought to provide such materials with a selective affinity for CO2 via the formation of ammonium carbamate species under anhydrous conditions and, in addition, via transformation to ammonium bicarbonate and carbonate species in the presence of water (eq 1).1-6 CO2
CO2/2H2O
2(∼RNH2) 98 (∼RNHCOO- +H3NR∼) 98 2(∼RNH3+ -O3CH) (1) The specific CO2 adsorption capacities reported for such amine-functionalized silica materials to date (up to 7.3 mass %)1,4,5,7-11 are not outstanding in comparison, for example, to zeolites (reported to adsorb in the vicinity of 4 mol of CO2/kg or 17% by mass at ambient temperature and pressure12). In our previous work, the materials prepared with the larger modifier (ED) typically led to hybrid materials with fewer tethers per unit substrate surface area, but with more N per unit mass and * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: 61 3 9905 4626. Fax: 61 3 9905 4597.
more N per unit product surface area. Consequently, materials with larger CO2 adsorption capacities per unit mass (at 20 °C) and larger CO2 adsorption capacities per unit surface area were prepared albeit with lower amine efficiencies. Thus, we were interested in investigating diethylenetriamine[propyl(silyl)](DT-) functionalized HMS (illustrated in Figure 1), the next homologue in the series, because the longer side chain and greater number of N atoms per tether offer the potential to achieve still higher CO2 adsorption capacities. The HMS-type substrate was previously chosen as it was thought that this highly mesoporous substrate would be useful for preparing products that would permit rapid gas diffusion to a large active surface domain. Each of the HMS materials was prepared via a neutral templating method.1 The neutral template approach13 was chosen for two reasons: (a) because it enables the recovery and recycling of the template employed in its synthesis and (b) because it is claimed that the resultant HMS materials contain thicker framework walls that impart greater thermal and hydrothermal stability. Moreover, because the template can be removed by solvent extraction, it is possible to prepare surfaces with higher surface silanol concentrations by avoiding exposure to calcination temperatures.1,14 Because flue gas contains water, the adsorption of CO2 on the amine-functionalized HMS products in the presence of water was also studied. It was found that the presence of water did not significantly affect the amount of CO2 adsorbed on the APfunctionalized HMS, but led to a greater heat of adsorption. This was attributed to the concurrent slow formation of bicarbonate species. The ED-functionalized HMS exhibited a lower CO2 adsorption capacity in the presence of water, but also with a higher heat of adsorption. This was explained in terms of poorer access of CO2 molecules to the amine sorption sites, which were already occupied by adsorbed water.1 Hyoshi et al.10 have separately considered the use of SBA15, a different high-surface-area mesoporous silica substrate with a regular periodic framework structure, to prepare AP-, ED-, and DT-functionalized mesoporous silica products as CO2
10.1021/ie050589g CCC: $33.50 © 2006 American Chemical Society Published on Web 02/25/2006
Ind. Eng. Chem. Res., Vol. 45, No. 8, 2006 2627
Figure 1. Surface structural representation of HMS and its aminopropylsilyl- (AP-), ethylenediamine[propyl(silyl)]- (ED-), and diethylenetriamine[propyl(silyl)]- (DT-) functionalized products. Table 1. Basic Preparation and Characterization Details for Substrate and Product Materialsa
material S2 S2-DT H2 H2-DT H2-DTL H5a H5a-DT H5b H5b-DT
vol of silane (mL g-1) 1.3 4.9 0.5 4.9 3.0
surface area (m2 g-1)
pore vol (mL g-1)
pore diam (Å)
He density (g mL-1)
567 342 909 599 659 762 484 1268 926
0.67 0.39 1.06 0.47 0.62 1.02 0.44 0.98 0.46
36.8 35.6 29.6 22.1
